In a packet communications system, data packets are transmitted from a transmitting node over one or more networks to a receiving node. For example, the transmitted data packets may be part of a video stream with one or more data packets required to reproduce a frame of the originally created video signal. In this example, the receiving node may be part of a video transcoding gateway which receives video packets of a video stream transmitted in one format (such as a MPEG 4 standard) and translates it into another format (for example H263 standard) for onward transmission to an end user.
The receiving node typically includes a buffer for storing data packets received at the receiving node and for releasing them to an application in the receiving node when called. Variations in the times of arrival of the data packets at the receiving node, typically called jitter delay, can occur due to network congestion, or timing drift. The jitter delays are typically random and can cause data packets to arrive at the receiving node out of sequence and/or with gaps or overlaps. The buffer in the receiving node which receives and stores the received data packets is arranged to smooth out the delays in receipt of the data packets due to jitter and is typically called a jitter buffer. For example, a jitter buffer enables video frames of a video stream to be played with reduced distortion due to jitter. The jitter buffer is typically defined by a buffer length which determines the number of data packets that can be stored in the jitter buffer at a given time and also a parameter known as buffer delay which is selected to correspond to a time period between when a data packet is received at the jitter buffer without delay and when it is released. The length and buffer delay of a jitter buffer is chosen so that the delays due to jitter are compensated for whilst ensuring the data packets are released from the jitter buffer to the application at the receiving node at the required rate and in the proper order to ensure that jitter is removed when the data packets are released. If the jitter buffer length is too small, then too many packets may be discarded or dropped which may lead to a reduction in quality when the packets are released. If the jitter buffer delay is too large, then the additional delay may lead to inefficient releasing of the data packets. For audio applications, the additional delay can lead to conversational difficulty.
Adaptive jitter buffers are known. For example, U.S. Pat. No. 7,346,005 describes a jitter buffer arrangement with adaptive playout of digital packet audio. The jitter buffer arrangement disclosed in this patent calculates jitter delay based upon comparing the arrival time of a current data packet with an expected receive time of the current packet, which is determined from the duration (packet length) and receive time of a previous packet. The difference in the arrival time and the expected receive time or jitter delay is maintained on a running average in real time and is used to set the buffer delay of the jitter buffer when appropriate to allow for proper playout of the received data packets.
U.S. Pat. No. 6,452,950 describes a similar arrangement for adapting the buffer delay based on an estimated variation of packet transmission delay derived from the times of arrival of stored packets using the Real Time Protocol (RTP) to achieve smooth data feed to an application without excessive delays.
U.S. Pat. No. 6,704,329 describes the use of a non-linear pre-filter prior to a low pass filter to remove the effect of network jitter from an error signal representing the difference between actual time of receipt and expected time of receipt of a data packet which is used to control the output data rate from the jitter buffer (that is the buffer delay).
Although these known adaptive arrangements can dynamically change buffer delay to take account of variations in jitter delay, these arrangements take account of variations from a substantially constant delay and so these arrangements cannot adapt quickly to significant changes in the overall transmission delay for example due to network delay changes, such as a change in transmission path, which cause rapid and significant changes to the time of arrival of the data packets at the receiving node.